Subtrace data not handled by any importer should be ignored.

When importing zip, gzip traces, the archive might contain eventData
which is neither json nor empty string. Trace-viewer doesn't know
how to handle that eventData and it throws no importer error.

Any eventData which is not json, should be treated as empty string
and should be handled by TraceModelEmptyImporter itself.

BUG= #613 
On second thought, can we just turn this into a boolean. isTraceDataContainer, default it to false and set to true in the few places we have a container?